The present invention relates to a sensor system for sensing a security or a breakage of large structures such as a bridge, a building, or an atomic power plant, and the like, more particularly, to a fiber optic sensor system for sensing strain of the structures which allows the strain of the structures to be sensed precisely in real time during the entire time of operation of the structure by using a wavelength swept fiber laser (often, referred to as xe2x80x9cWSFLxe2x80x9d) as a light source and using a fiber Brag grating (often, referred to as xe2x80x9cFBGxe2x80x9d) sensor as a fiber optic sensor (FOS).
Currently, there have been many researches on the applications of using fiber optic sensors in smart composite structures that are composite structures combined with sensors. In this regard, unlike a conventional security inspection method for large structures, a security diagnosis technology has been developed which enables a security or a breakage of the smart composite structures to be sensed precisely in real time for during the entire time of operation of the structures. This technology using fiber optic sensor contributes to prevention from a ruin of the structures such as a bridge or an atomic power plant which may suffer great damage upon the breakage of the structures, and reduction in a cost incurred due to maintenance and repair of the structures.
The smart composite structures are composed of a sensor system, a brain system, and actuating system. The sensor system is adapted to sense the change of an external environment. The brain system is adapted to process the sensed information, and includes a microprocessor having a signal processor and database for characteristics of the structures installed therein. The actuating system is adapted to actively cope with the sensed change of the external environment, and comprises a piezoelectric ceramic, ER/MR fluid, or a Shaped Memory Alloy (SMA).
In the meantime, the sensing system may comprise a semiconductor sensor, a metallic thin film sensor, a piezoelectric sensor, or a fiber optic sensor, etc. In the case of constructing the sensing system by using the fiber optic sensor, it has several advantages in that it is not affected by the electric magnetic field, is very extensive in the range of an operating temperature, and has an extremely small diameter. In addition, the fiber optic sensor is flexible, so it is possible to easily construct a sensor having a size that a user wants. Further, the fiber optic sensor can provide a high resolution and transmit a large quantity of information. As a result, its application range is being extended increasingly.
The fiber optic sensor can be classified into an amplitude sensor, a polarization sensor, and an interferometic sensor according to a sensing principle. More specifically, the fiber optic sensor comprises a sensor implemented by a method which utilizes information on whether or not light can be transmitted due to breakage of an optical fiber""s end, a sensor implemented by a method utilizing polarization of light, and a sensor implemented by a method utilizing interference of light such as Mach Zender, Michelson, and Fabry-Perot interferometer fiber optic sensors. Among these sensors, the interferometer fiber optic sensor using interference of light, which is most widely used to measure the strain or composite structures, is also adapted to measure strain behavior from an interference signal due to a difference between light paths caused by a strain of structures. However, such interferometic fiber optic sensors is not efficient for the application of a multiplexing used to measure many points simultaneously, and involves many problems in the process of processing an output signal from the sensors. Therefore, in these days, FBG sensors have been developed newly and are now used which are able to simply measure disturbances such as strain behavior and temperature by measuring a variation in a reflection wavelength.
The FBG sensors are formed in such a fashion to provide regions where an index of refraction is changed in a core of an optical fiber, i.e., Bragg gratings are formed in the fiber optic core at predetermined, spaced apart intervals. At this point, wavelength components that are determined based on an effective index of refraction of the FBG and the interval between the gratings are reflected from each of the FBG, and the remaining wavelength components are transmitted. Therefore, if disturbances such as strain, and the like are applied to the FBG, the wavelength of the reflected light varies, and it is possible to measure strain behavior of the structures through a measure of an amount of the changes in the wavelength of the light reflected by the respective Bragg Gratings. Further, if the Bragg Gratings are configured such that they are not overlapped from each other even when the Bragg Gratings are subjected to strain by virtue of making a center wavelength of the light reflected by the Bragg Gratings different, it is possible to inscribe a plurality of FBG sensors into a length of optical fiber, and hence a simultaneous multi-point detection technique of strain can be easily implemented. Also, it is possible to measure an absolute amount of strain where strain is distinct from compression, and since an output signal is associated with only the wavelength of light, a fiber optic sensor system can be implemented easily irrespective of a change in the intensity of light. In addition, since the output signal is linear with respect to strain, the FBG sensors maintain nearly the intensity of an optical fiber itself, so an intensity characteristic is good.
There are well known conventional methods for demodulating a signal outputted from FBG sensors having such advantages. That is, either a Michelson interferometer or a Mach Zender interferometer is configured as a demodulator for demodulating the signal outputted from FBG sensors, or a fiber Fabry-Perot filter is used as the demodulator. However, for such a demodulator system configured with either the Michelson interferometer or the Mach Zender interferometer, there is no an absolute reference of the demodulator system, and a minute difference in an optical path is required, so the demodulator system is significantly difficult to fabricate. Furthermore, the most disadvantage of the demodulator system is that if a multiplexing method of inscribing a plurality of FBG sensors into a length of optical fiber is employed, a considerably complex signal processing process becomes necessary. Also, in case of such a demodulator system using the fiber Fabry-Perot filter as a demodulator, the multiplexing method is possible, but since a resolution is determined by a bandwidth of the fiber Fabry-Perot filter, a resolution of the system is deteriorated. Thus, in order to improve the resolution, a high-priced fiber Fabry-Perot filter having a narrow bandwidth is necessary, which results in an increase of a cost. To resolve this problem, there has been developed and is used a fiber optic sensor system using, as a light source, a Wavelength Swept Fiber Laser (hereinafter, interchangeably referred to as either xe2x80x9cWSFLxe2x80x9d or xe2x80x9cWavelength Swept Fiber Laserxe2x80x9d) which does not require a separate demodulator system since a light source itself outputs information about a wavelength such that the information is associated with a time region. Such a system has an advantage in that it has a high output power of light and a good resolution. However, in case of this system, a non-linearity may be exhibited in the system due to a non-linearity of the fiber Fabry-Perot filter, and there may occur an error when measuring strain on a structure.
Therefore, the present invention has been made in view of the above-mentioned problems and to resolve several problems which may be occurred when measuring strain on a structure due to a non-linearity of the FBG sensor system using a WSFL as a light source, and it is an object of the present invention to provide a fiber optic sensor system for sensing strain of a structure by which a Fabry-Perot etalon is added to a conventional FBG sensor system so that a coordinate is set with an output signal of light reflected from the Fabry-Perot etalon at a wavelength region to correct the non-linearity of the FBG sensor system, thereby allowing an abnormal state of the structure to be sensed without an error in real time.
According to one aspect of the present invention, there is provided a fiber optic sensor system for sensing strain of structures by using a WSFL as a light source and using a FBG sensor as a Fiber Optic Sensor, comprising:
a reference FBG sensor installed within the structure so that it is not subject to strain, the reference Fiber Bragg Grating sensor functioning as a reference for referring to information on a wavelength;
a Fiber Bragg Grating sensor array including at least one Fiber Bragg Grating sensor embedded in the structure for sensing the strain of the structure, the Fiber Bragg Grating sensor array being branched from the reference Fiber Bragg Grating sensor by a coupler;
a strain rate measuring coordinate signal-outputting means branched from a Fiber Bragg Grating sensor group consisting of the reference Fiber Bragg Grating sensor and the Fiber Bragg Grating sensor array through a coupler, the strain rate measuring coordinate signal-outputting means adapted to output an optical signal having a uniform wavelength; and
an optical detector for detecting output signals of the optic fiber Bragg sensor group and the strain rate measuring coordinate signal-outputting means.